Myotonic dystrophy (DM) is the most common adult onset muscular dystrophy. DM type 1 (DM1) is a multi-systemic neuromuscular disease caused by a CTG expansion in the 3'untranslated region of the DMPK gene. The repeat-containing RNA retained in the nucleus sequesters the MBNL1 splicing factor and leads to up-regulation of the CUGBP1 splicing factor. Misregulated alternative splicing plays a critical role in the disease mechanism and has been directly attributed to two disease symptoms, myotonia and insulin resistance. The ability of repeat-containing RNA to induce other disease symptoms such as skeletal muscle wasting, the leading cause of death in patients, is not known. In Specific Aim 1, the prevalence of splicing changes and alterations in the levels of splicing regulators in different muscular dystrophies will be determined using tissue from six muscular dystrophy mouse models. In Specific Aim 2, the role of CUGBP1 in skeletal muscle wasting will be investigated. CUGBP1 is aberrantly up-regulated in DM1 patients and a DM1 mouse model with a strong wasting phenotype. To determine if increased CUGBP1 is necessary for wasting in this DM1 mouse model, CUGBP1 will be knocked down by delivery of shRNA via an adeno-associated virus. Then, the muscle integrity will be tested by exercise tests, muscle weight, and muscle histology. To determine if increased CUGBP1 is sufficient to induce wasting, mice with doxycycline-inducible and skeletal muscle specific expression of CUGBP1 have been generated and will be examined for muscle function and wasting. In Specific Aim 3, the involvement of PKC in CUGBP1 up-regulation will be determined. In DM1 heart tissue and an inducible DMl heart model, CUGBP1 up-regulation in response to expanded CUG repeats occurs through a PKC-dependent phosphorylation event resulting in hyperphosphorylated and stabilization of CUGBP1. PKC activity and CUGBP1 hyperphosphorylation will be determined in skeletal muscle from the DMl mouse model. PKC inhibition by Bis-IX treatment will be explored as a potential therapeutic approach to inhibit CUGBP1 up-regulation and prevent muscle wasting in the DM1 mouse model. These proposed experiments address a very important question in the field about the cause of muscle wasting in myotonic dystrophy. The mechanisms at play may be more broadly applicable to other neuromuscular diseases and trinucleotide repeat diseases. Public health statement: The primary cause of death in myotonic dystrophy patients is due to the severe skeletal muscle wasting. This proposal aims to determine the cause of skeletal muscle wasting using a myotonic dystrophy mouse model and potentially prevent the wasting using a chemical inhibitor.